1. Field of the invention
The present invention relates to a display system, optimized for night vision and for day vision, for the helmet of a vehicle driver or pilot. It applies in particular to the helmets of pilots of combat helicopters or military aircraft.
2. Discussion of the Background
A helmet display system is a device for displaying the information superimposed on the view of the landscape.
Presenting the pilot or driver with appropriate information, for example in the form of symbols, makes it possible to aid driving or piloting, and navigation. In particular for combat vehicles, a reticule provides an aid for aiming a weapon.
Inside the helmet, a symbol generator comprises a cathode-ray tube on whose screen an image of the symbols is formed. The helmet also comprises an optical relay system for conveying this image to a combiner which presents the conveyed image superimposed on the view of the landscape.
The combiner is generally formed by a part of the visor which has been surface-treated to reflect an image to the eye of the driver or pilot while allowing him to observe the landscape by day through the visor.
In order to permit night vision as well, helmet display systems comprise an integrated light intensifier. The image of the night-time scene is received by an objective lens and is then intensified by the light intensifier.
The light intensifier comprises a photocathode at its input and a cathode-ray screen at its output. The photons received on the objective lens are converted into electrons by the photocathode. The light intensifier accelerates and multiplies these electrons so as to finally form an intensified image on its cathode-ray screen.
The intensified image of the landscape, on the one hand, and the image of the symbols, on the other hand, are mixed by a mixer to form a single image which is conveyed to the combiner. The combiner reflects this image and superimposes it on the very dark night-time scene.
However, the state of the art as regards a light intensifier which can be integrated with a helmet leads to a low luminosity for the intensified image. The mixer thus favours the intensified image over the image from the symbol generator, so that the pilot or driver sees an intensified image of the landscape without being dazzled by the superimposed symbols which are too bright.
The mixer is typically such that the luminosity of the mixed image is made up of 75% of the luminosity of the intensified image and 25% of that of the symbol image.
For day vision with a helmet having a display system with integrated light intensifier, the driver or pilot turns the intensifier off. An intensified image is not formed, and the mixer transmits only the symbol image; this image is conveyed to the combiner and allows the driver or pilot to see the symbols superimposed on the daytime landscape. The objective lens and the light intensifier are then superfluous.
Thus, during a day mission, a helmet with a display system having an integrated light intensifier makes the driver or pilot have to wear elements on his head which are heavy and bulky while fulfilling no purpose.
Furthermore, the mixer attenuates the image from the symbol generator by transmitting about a quarter of its luminosity, and this attenuated image is superimposed by the combiner on the view of the very bright daytime landscape. The luminosity of the symbols is insufficient in comparison with that of the landscape, and the driver or pilot does not see these symbols clearly during day missions.
These two drawbacks are found to be very serious for known helmet display systems which provide for day vision and night vision.
The object of the invention is for the driver or pilot to see the superimposed symbols clearly both in day vision and in night vision.
To this end, the invention relates to a helmet comprising a night vision system, an image generator, an optical mixing system for mixing the light rays output by the night vision system and the light rays output by the image generator, and a fixed optical system downstream of the optical mixing system, characterized in that it includes:
a first optical mixing system having a first transmission coefficient for the rays output by the image generator,
a substitute optical system having the same optical characteristics as the optical mixing system for the rays output by the image generator, but with a second transmission coefficient which is greater than the first,
and a housing upstream of the fixed optical system, for accommodating either the optical mixing system or the substitute optical system in such a way that they can be removed.
A helmet according to the invention has a modular architecture with a night module, comprising the optical mixing system and the night vision system, and a day module comprising the substitute optical system.
The night module is optimized for night missions, while the day module is optimized for day missions.
The driver or pilot changes the module himself according to his requirements in terms of a display device, depending on the type of mission and depending on the ambient lighting conditions during the mission.
In night vision with the night module and the night vision system turned on, the driver or pilot sees symbols superimposed on the intensified image of the landscape which is provided by the light intensifier of the night vision system. The luminosity of these symbols, and that of the intensified image, are compatible with comfortable viewing on the part of the driver or pilot.
In day vision, the driver or pilot removes the night module then places the day module in the location thus freed. The latter module ensures that symbols superimposed on a very bright landscape can be seen clearly.
Furthermore, the day module is much less encumbering for the head of the driver or pilot than the assembly comprising the optical mixing system and the night vision system. This decrease in weight allows a substantial reduction in the discomfort and fatigue suffered by the wearer of the helmet during day missions.
The replacement of one module by the other is carried out by the driver or pilot himself when he is wearing the helmet, by manual extraction of the module in place and straightforward insertion of the other module. Adjustment is automatic.
The replacement takes little time and can be done in flight because the direct view of the landscape through the combiner is maintained when there is no module inserted in the housing.
For binocular vision, the helmet has two complete sets of equipment, one for the right eye and the other for the left eye; i.e. two night modules and two day modules.